The present invention relates to a cylinder linear unit for use in an internal combustion engine, and more particularly, to a cylinder linear unit which is adapted to be cast in a cylinder block made of an aluminum alloy.
Recently, cylinder blocks made of an aluminum alloy have been used in mass produced automobile engines. As a material for cylinder liners, a non-aluminum alloy such as cast iron or ceramics is generally used.
FIG. 6 is a plan view of an cylinder block of a four-cylinder engine. The cylinder block is manufactured by casting cylinder liners 2 made of a non-aluminum alloy in a cylinder block 1 made of an aluminum alloy. The cylinder block 1 must be provided with a water jacket 5. When casting the cylinder block, it is difficult to precisely position each of cylinder liners 2 in place, so that it is complicated and costly to produce a metallic mold for the casting liners.
Japanese Utility Model Laid-Open Application Sho 61-107461 discloses that, as shown in FIG. 6, the thickness of an aluminum alloy base material interposed in interspace D between adjacent cylinder liners 2 should be substantially equal to the wall thickness of the cylinder 2 to prevent deformation of the cylinder liner 2 during thermal expansion.
When the interspace D between adjacent cylinder liners 2 is reduced, however, the flow of molten metal becomes worse during the casting operation of large-sized castings such as a cylinder block and defects in casting such as cleavage and cavities in molten metal are liable to occur. This is applicable to an engine having a reduced bore pitch.
In order to eliminate the above-mentioned disadvantages of separately casting the cylinder liners 2 one by one, an integrally combined cylinder liner unit 10 made of cast iron as shown in FIG. 5 is employed. The cylinder unit 10 integrally combined by connecting bores 11 with connectiong portions C is cast in a cylinder block 1 made of an aluminum alloy, as shown in FIG. 4.
In the case where an integrally combined cylinder liner unit made of cast iron is employed, the material of the cylinder block located around the liner (which is an aluminum alloy) does not melt and combine with the cast iron of the liner. Consequently, the cylinder block is separated from the cylinder liner at portions A and B shown in FIG. 4 to open outward, resulting in decrease in the strength of the cylinder liner and leakage of water and combustion gas.
In addition, since a connecting portion C exists in the cylinder liner, the radial wall thickness of the cylinder liner around each bore 11 is not uniform at every circumferential point of the bore. Consequently, when temperature of the cylinder liner rises during running of an engine, the thermal expansion of the cylinder liner is not uniform at every circumferential point thereof and the bore 11 loses its roundness.
As a result, the output of the engine decreases due to gas leakage. Additionally, there is insufficient compression and an increase in oil consumption due to leakage of lubricant oil.